1. Field of the Invention
This invention generally relates to power conversion systems, more particularly to non-isolated power conversion systems that use multiple switching power converters.
2. Description of the Prior Art
Isolated and non-isolated power conversion systems are known. Isolated power supplies generally use a transformer for isolating an input power stage from an output power stage through primary and secondary windings. Non-isolated power conversion systems usually use cascaded switching power converter stages associated with one or more switching cycles.
Known non-isolated power converters have been used in AC-DC, AC-AC, DC-AC, and DC-DC applications. Examples of such converters include buck converters, boost converter and buck-boost converters that can be implemented using various switching power conversion topologies. In such topologies, the input/output conversion ratios are determined according to duty cycles associated with the switching cycles. For example, a boost converter is a step-up power converter having a voltage conversion ratio that is greater than 1. On the other hand, a buck converter is a step-down converter having a voltage conversion ratio that is less than 1. In other words, the input voltage of the boost converter is always less than or equal to the output voltage, whereas, the input voltage of the buck converter is always greater than or equal to the output voltage.
The buck converter topology has been extensively used in various DC-DC applications. In fact, the non-isolated voltage-regulation modules (VRM) of today's microprocessor power supplies are almost exclusively implemented with the buck topology. FIG. 1 shows a prior art buck converter that uses a single-inductor and a switching stage to provide an output voltage that is less than the input voltage. In this prior art converter, the output-to-input conversion ratio is equal to the duty cycle associated with the switching cycle.
This prior art buck converter and its known variations exhibit satisfactory performance in low-current applications. In high-current applications, however, it may be desirable to implement a multi-stage buck converter topology that comprises multiple switching stages and inductors. One such multi-stage buck converter used in high-current applications is shown in FIG. 2. Also, it is known to use multi-stage buck converters in low output voltages applications in order to improve conversion efficiency when conduction loss under a single-stage buck converter topology is severely degraded.
The buck converter of FIG. 2 uses buck topology in parallel. This topology is often used in high-current VRM applications to reduce current stress by operating more than one buck converter in parallel. The switching instances of each switch are interleaved, i.e., phase shifted, for 180 degrees. With such interleaving, the output current ripple is reduced and, consequently, the size of the output filter capacitor is minimized. Because the duty cycle of the conventional buck converter is proportional to the conversion ratio of input/output voltage in applications that use high switching frequencies for providing high conversion ratios, the turn-on periods of the switches are extremely short. Consequently, extremely narrow switch activation pulses are necessary for maintaining very short duty cycles. Generating very narrow turn-on switch activation control signals, however, is difficult because of parasitic components that are associated with the switching devices and the switch activation circuit.
In addition, a conventional buck converter in applications that require a high-voltage conversion ratio suffers from a serious efficiency degradation. This is because the blocking voltage of the switches in a conventional buck converter is equal to its input voltage. Thus, the voltage rating of the switching devices should be higher than the input voltage. Usually, high-voltage switching devices are more expensive and have greater conduction losses in comparison with low-voltage-rated switching devices. The efficiency of the conventional buck converter is further degraded by a severe switching loss. This is because the switching loss is approximately proportional to the square of the voltage across the switch during the instances when the switch is turned on and turned off.
Other known prior art approach to non-isolated power converters used in applications that require delivering high-voltage output from low-voltage input is a boost converter. Because the duty cycle of the conventional boost converter should be maximized to provide a very large conversion ratio of input/output voltage, the turn-on periods of the switches are extremely long. Consequently, extremely long switch conduction period increases conduction losses and lowers converter efficiency. Therefore, there exists a need for a power conversion system that includes multiple power converters to provide efficient power conversion, even at high conversion ratios.